In recent years, lithium secondary batteries that can be small and lightweight and can have a high capacity have been widely used as power sources for cellular phones. Moreover, nonaqueous electrolyte secondary batteries have been recently attracting attention as power sources for high-power-demanding applications, such as electric power tools and electric vehicles. Furthermore, demands for higher-capacity batteries are increasing in applications that require higher-power nonaqueous electrolyte secondary batteries.
With the increase in capacity, it is increasingly important to provide the safety of batteries. In particular, nonaqueous electrolyte secondary batteries, which are characterized by having higher energy densities than those of other batteries, can lead to a rapid increase in temperature triggered by an increase in the temperature of batteries due to misuse of batteries. Thus, a nonaqueous electrolyte secondary battery is required to have higher thermal stability.
To improve nonaqueous electrolyte secondary battery, the following attempts are made:
(1) a report that surfaces of lithium-containing transition metal oxide particles are coated with a lithium compound (PTL 1 described below), and
(2) a report that 15% or more of an apparent surface of a powdery metal oxide constituting an active material is covered with a carbon material having a specific surface area of 150 m2/g or more in such a manner that the carbon material has a thickness of 0.01 μm to 0.3 μm (PTL 2 described below).